CN112321757A

CN112321757A - Regulator control in high pressure polyethylene production

Info

Publication number: CN112321757A
Application number: CN202011250546.4A
Authority: CN
Inventors: H·A·拉曼斯; F·K·韦尔卢藤
Original assignee: ExxonMobil Chemical Patents Inc
Current assignee: ExxonMobil Chemical Patents Inc
Priority date: 2015-08-31
Filing date: 2016-07-26
Publication date: 2021-02-05
Also published as: EP3344662A1; KR101995680B1; WO2017039877A1; CN107922516A; KR20180031747A; US20180244813A1; US10570226B2; MY184536A

Abstract

The present invention relates to regulator control in the production of high pressure polyethylene. Methods and systems for controlling the concentration of a regulator in an ethylene polymerization reactor are disclosed.

Description

Regulator control in high pressure polyethylene production

The application is a divisional application of Chinese patent application with the application number of 201680049541.0, the application date of 2016, 26 months and 26 days, and the name of the invention of regulator control in the production of high-pressure polyethylene.

Cross Reference to Related Applications

This application claims the benefits of U.S. s.N.62/212377 filed on day 8/31 of 2015 and EP15188169.5 filed on day 10/2 of 2015, the disclosures of which are incorporated herein by reference in their entireties.

Technical Field

The invention relates to a method for controlling a regulator in a high-pressure polyethylene production process.

Background

High pressure reactor polymerization equipment converts relatively low cost olefin monomer, typically ethylene, optionally in combination with one or more comonomers such as vinyl acetate, into valuable polyolefin products. Such methods using oxygen or organic free radical initiators (especially peroxide initiators) are known in the art and have been used in the industry for a long time. The polymerization is carried out at relatively high temperatures and pressures and is highly exothermic. The polymer formed is a Low Density Polyethylene (LDPE), optionally containing comonomers.

The high pressure polymerization process is carried out in an autoclave or a tubular reactor. In principle, autoclave and tubular polymerization processes are very similar, except for the design of the reactor itself. The plant typically uses two main compressors, each having multiple stages, arranged in series to compress the monomer feed. The primary compressor provides initial compression of the monomer feed and the secondary compressor increases the pressure generated by the primary compressor to a level at which polymerization occurs in the reactor, which is typically from about 210 to about 320MPa for tubular reactors and from about 120 to about 200MPa for autoclave reactors.

Regulators or chain transfer agents are often used in high pressure polymerization processes to reduce molecular weight and narrow the molecular weight distribution. It is generally known to add a conditioning agent either in the secondary compressor suction or in the primary compressor purge. However, the addition of a modifier in the secondary compressor can lead to premature thermal polymerization and build-up of polymer in the compressor line, which in turn can lead to fouling. Fouling can completely block the gas flow lines in the rest of the process, which can cause undesirably high pressure drops, reduced throughput, and poor pumping efficiency in the secondary compressor.

It is also known to add the moderator directly to the reactor at a location, but controlling the concentration of the moderator within the reactor is challenging, and poor control can also lead to fouling. Various methods are known to control the concentration of regulators or chain transfer agents within the reactor. U.S. patent No.6899852 discloses a tubular reactor process for obtaining polymers with low haze. The monomer feed stream to the reactor is separated into a transfer agent-rich stream and a transfer agent-poor monomer stream, and the transfer agent-rich stream is supplied upstream of at least one reaction zone receiving the transfer agent-poor monomer stream. The transfer agent-depleted monomer stream has 70 wt.% or less transfer agent as compared to the transfer agent-enriched stream to achieve depletion of chain transfer agent concentration in the downstream reaction zone.

When chain transfer agents with high chain transfer constants are used in the known process, the concentration of residual transfer agent will be rather low towards the end of the reactor. This can lead to the production of high molecular weight polymers, leading to reduced heat transfer and fouling. Reactor descaling is performed to restore heat transfer so that the process can be operated within the temperature window required for safety and optimum production rates. Reactor descaling will typically involve periodic removal of accumulated polymer by mechanical (e.g., hydraulic cleaning or auger) or chemical (e.g., polymer skin material) means. The descale adds expense and complexity to the process and creates down time. Other background references include US2005/192414, WO2014/046835, WO2011/128147, WO2012/084772, WO2015/100351 and EP 2636690A.

There is a need for a process that allows for better control of the regulator or chain transfer agent concentration to mitigate fouling and minimize the need for reactor fouling.

Summary of The Invention

The present invention relates to a process for controlling the concentration of a regulator in an ethylene polymerization reactor, the process comprising: the ethylene monomer was compressed to a pressure of 1000-3000 bar; introducing the compressed ethylene monomer into a reactor; introducing a modifier into the reactor front end in at least one front stream; and introducing the moderator into the side of the reactor in at least three side streams spaced along the length of the reactor.

The present invention also relates to an ethylene polymerization reactor system comprising: first and second compressor stages for compressing monomer; a reactor; at least one front flow for introducing a moderator into the front end of the reactor; and at least three side streams spaced apart along the length of the reactor for introducing the moderator into the sides of the reactor.

The process and system of the present invention may be carried out in any suitable reactor system. In a preferred embodiment of the invention, the reactor is a tubular reactor or an autoclave reactor.

Brief description of the drawings

Fig. 1 schematically illustrates an ethylene polymerization plant or system according to one embodiment of the present invention.

Detailed Description

In the manufacture of high pressure polyethylene, it is generally known to add a modifier in the suction of the secondary compressor or in the primary compressor to act as a chain transfer agent and thus control the molecular weight of the ethylene product. However, the addition of a modifier to the secondary compressor can lead to premature thermal polymerization and fouling within the secondary compressor and intercooler.

It is also known to add the regulator directly to the reactor, but controlling the regulator concentration within the reactor has been challenging. In one known tubular reactor process, the moderator is added at two locations: a front flow at the reactor inlet and a side flow at the side of the reactor. In this method, the amount of modifier is divided equally between the two streams. In another known tubular reactor process, as disclosed in U.S. patent No.6899852, the monomer feed stream fed to the reactor is divided into a transfer agent-rich stream and a transfer agent-poor monomer stream, and the transfer agent-rich stream is fed upstream of at least one reaction zone that receives the transfer agent-poor monomer stream. In both processes, depletion of the regulator concentration occurs in the reaction zone downstream of the regulator entry point. In addition, the moderator is often depleted at different rates, which further results in concentration variations along the length of the reactor.

Since the moderator is consumed along the length of the reactor, the residual moderator concentration in the reaction zones downstream of the moderator entry point and in the final reaction zone can become considerably lower in the processes of the prior art. This problem is exacerbated when regulators with high chain transfer activity are used, for example aldehydes such as propionaldehyde or acetaldehyde, since regulators with high chain transfer activity are depleted more quickly. When the modifier concentration drops too low, higher molecular weight polymer is formed, which can lead to fouling and out of specification polymer production within the reactor system.

The limited number of modifier entry points in the prior art processes also typically requires the addition of higher amounts of monomer at each entry point. This can result in locally high concentrations of modifier along the length of the reactor. These local high concentrations can be problematic, particularly when saturated regulators such as methane, propane, butane and others are used, as they result in the formation of new short chain molecules which can also lead to out-of-specification products and fouling.

The method and system of the present invention enable improved control of the concentration of the moderator across the length of the reactor. The injection points for applying the moderator to the reactor in the at least one front flow and the at least three side flows allow the moderator concentration profile to be adjusted more easily. This additional injection point compared to prior art methods may also reduce the amount of conditioning agent that has to be added at any one injection point, which can avoid undesired local high concentrations of conditioning agent.

In the method and system of the present invention, multiple locations of injection of the modulator may be supplied by one modulator pump with flow controllers for different locations, or using separate modulator pumps. In addition, the modifier may be premixed with the solvent, monomer feed or initiator prior to addition to the reactor.

The amount of moderator fed to the reactor through each stream can be readily adjusted and adjusted to take into account desired process operating parameters and limitations. The at least one front stream can contain from about 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, or 45 wt% of the modifier at the low point to about 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, or 80 wt% of the high point, based on the total weight of the material supplied through the at least one front stream, and suitable ranges include any upper or lower limit. Each sidestream individually may contain from a low point of about 0.1 wt%, 10 wt%, 20 wt%, 30 wt% or 40 wt% to a high point of about 20 wt%, 30 wt%, 40 wt%, 50 wt% or 60 wt% of the conditioning agent, based on the total weight of materials fed through the sidestream, and suitable ranges include any upper or lower limit. The amount of conditioning agent supplied individually by each side stream may differ from the amount supplied by each other side stream by less than 15 wt%, 10 wt%, 5 wt%, 3 wt%, 2 wt%, 1 wt% or 0.5 wt%. Furthermore, the amount of conditioning agent supplied by at least one preceding stream may differ from the amount supplied by each other stream by less than 15 wt%, 10 wt%, 5 wt%, 3 wt%, 2 wt%, 1 wt% or 0.5 wt%.

The invention enables better control of the regulator concentration in the reactor. The reactor includes at least one reaction zone, and may include a plurality of reaction zones. Each reaction zone is optionally followed by a cooling zone. The number and location of moderator injection points along the reactor can be adjusted to control the moderator concentration in the or a particular reaction zone. For example, the concentration of the modifier in any one or each reaction zone can be maintained at or above a minimum amount, such as at or above 0.1 wt%, 0.5 wt%, 0.75 wt%, or 1.0 wt%, based on the total weight of materials in that reaction zone. Additionally or alternatively, the concentration of the moderator in the last reaction zone can be maintained at or above 0.1 wt%, 0.5 wt%, 0.75 wt%, or 1.0 wt%, based on the total weight of the materials in the last reaction zone.

Since the thermal polymerization rate is generally increased in the presence of free radicals, free radical scavengers may also be used. The free radical scavenger may be added to the process via the feedstock and the lubricating oil. For example, in a process for making a vinyl acetate-based polymer, the vinyl acetate monomer can comprise a hydroquinone radical scavenger. The vinyl acetate monomer may contain 3-30ppm, 3-24ppm, 3-20ppm, 14-30ppm, or 14-24ppm hydroquinone. In another example, butylated hydroxytoluene (2, 6-di-tert-butyl-4-methylphenol or "BHT"), or other derivatives containing butylated hydroxytoluene units, may be used as free radical scavengers. The BHT may be present in the lubricating oil used in the secondary compressor cylinder and forms a film on the cylinder surfaces which prevents the formation of polymers on these surfaces. The amount of BHT present in the lubricating oil is typically from 1000ppm to about 6 wt.%, based on the total amount of the lubricating oil. Higher or lower amounts may be selected depending on, among other factors, the activity of the comonomer present in the process stream.

Process for producing ethylene polymer

Figure 1 is a schematic representation of a polymerisation plant 1 comprising an ethylene feed line 2 which feeds fresh ethylene to a primary compressor 3. The primary compressor 3 functions to pressurize fresh ethylene (make-up ethylene) to the pressure of the ethylene recycle system for feeding to the secondary compressor. The primary compressor may be a single compressor that compresses the ethylene alone, or it may be two or more compressors in series or parallel that are combined to compress the ethylene to the pressure of the ethylene recycle system. In some existing ethylene reactor plants, the ethylene discharged from the primary compressor is split into two streams (not shown), one stream is combined with recycled ethylene and fed to the suction of the secondary compressor, and the other stream is injected into the ethylene/polymer mixture downstream of the high pressure let down valve, thereby providing rapid cooling of the ethylene/polymer mixture prior to entering the product separation unit.

The ethylene discharged from the primary compressor 3 flows via the line 4 with valve 4a to the line 6a and then to the secondary compressor 5. Recycled ethylene is also fed to the secondary compressor 5 from the high pressure recycle system 16 via line 6 b. The secondary compressor compresses the ethylene to a pressure of at least 1000bar to be fed to the reactor 9. The secondary compressor 5 is typically a single motor driven unit, but may alternatively comprise two or more compressors in series or in parallel driven by separate motors (not shown). Any configuration of compressor is intended to be within the scope of the present disclosure as long as the configuration is suitable for compressing ethylene from the ethylene pressure at which it leaves the primary compressor 3 to the desired reactor pressure of 1000bar to 3000 bar.

The secondary compressor 5 discharges compressed ethylene in four

streams

8a, 8b, 8c and 8 d. Stream 8a can comprise about 20%, about 33%, about 50%, or another amount of the total ethylene flow. Stream 8a may be heated by a steam jacket (not shown) before entering the front end of reactor 9. The three remaining

ethylene side streams

8b, 8c and 8d each enter the reactor as a side stream and may be cooled before entering the reactor.

The reactor 9 has an initiator pumping station 11 for injecting initiator into the reactor via

initiator streams

11a, 11b and 11 c. In the reactor shown in FIG. 1 having multiple reaction zones, each initiator inlet defines the beginning of a reaction zone. Thus, the initiator stream 11a defines the beginning of the first reaction zone. As initiator is consumed in the reaction zone, the polymerization rate decreases. Additional initiator inlets are added downstream to form additional reaction zones. The injection of initiator causes an exothermic temperature rise downstream of the inlet, which is removed by cooling. This cooling can be achieved through the reactor wall via a cooling jacket (not shown) mounted to the reactor 9 and by means of a cooling liquid and/or a downstream cooling monomer supply. Generally, each entry of cold monomer defines the end of the reaction zone and the beginning of the cooling zone. The cooled ethylene side stream 8b thus defines the beginning of the first cooling zone. Likewise, initiator stream 11b defines the beginning of the second reaction zone and ethylene side stream 8c defines the beginning of the second cooling zone.

The reactor 9 also has a moderator pumping station 10 for injecting moderator into the reactor via a moderator front flow 10a and moderator side flows 10b, 10c and 10 d. The regulator is supplied from the regulator pumping station 10 via a flow controller (not shown) to regulate the amount of regulator supplied through each stream. In an embodiment of the invention, the initiator and the regulator may be premixed (not shown) and fed together through the front flow 10a and the three

side flows

10b, 10c and 10d, which eliminates the need for separate initiator pumping stations 11 and

flows

11a, 11b and 11 c. Optionally, fresh moderator can also be supplied to the reactor system, from the moderator pump 6 via a conduit 6c to the discharge or suction of the second stage of the secondary compressor 5.

The tubular reactor terminates in a high pressure let down valve 12. A high pressure let down valve 12 controls the pressure in the tubular reactor 9. Immediately downstream of the high pressure blowdown valve 12 is a product cooler 13. After entering the product cooler 13, the reaction mixture phase separates. It exits into the high pressure separator 14. The overhead gas from the high pressure separator 14 flows into a high pressure recycle system 16 where unreacted ethylene is cooled and returned to the secondary compressor 5.

The polymer product flows from the bottom of the high pressure separator 14 into the low pressure separator 15, which separates almost all of the remaining ethylene from the polymer. The ethylene is transferred to a flare (not shown) or purification unit (not shown) or it is recycled to the primary compressor 3. The molten polymer flows from the bottom of the low pressure separator 15 to downstream processing, which may be, for example, an extruder (not shown) for extrusion, cooling, and pelletizing.

The proportion of total ethylene entering the reactor 9, whether in the main feed stream 8a or as a

side stream

8b, 8c or 8d, which is converted to polymer before leaving the reactor 9, is referred to as conversion. In one embodiment of the invention, the conversion may be 30% to 40% and alternatively at least 35%. Conversions above 40% are feasible, but are not preferred, in part because the viscosity of the reaction mixture increases with its polymer content, which in turn results in an increase in the pressure drop required to maintain the necessary flow rate. The density of the ethylene polymer product made according to the present invention may be from 0.913 to 0.936g/cm³(as measured by ASTM D1505) and a melt index of 0.1 to 20dg/min (as measured by ASTM D1238). For example, the density of the ethylene polymer obtained from the process according to the invention may be from 0.915 to 0.920g/cm³And a melt index of 2 to 6 dg/min.

The process herein can be used to make ethylene homopolymers and copolymers, such as ethylene-vinyl acetate copolymers. Typically, the comonomer(s) will be pressurized and injected into the secondary compressor at one or more points. Other possible comonomers include propylene, 1-butene, isobutene, 1-hexene, 1-octene, other lower alpha-olefins, methacrylic acid, methyl acrylate, acrylic acid, ethyl acrylate and n-butyl acrylate. Reference herein to "ethylene" should be understood to include ethylene and comonomer mixtures, except where the context implies otherwise.

Initiator

As used herein, the term "initiator" refers to a compound that initiates the free radical ethylene polymerization process. Suitable initiators for use in the present invention include, but are not limited to, organic peroxide initiators. The peroxide is for example pure peroxide. Additional examples of suitable initiators include peresters including, but not limited to, bis (2 ethylhexyl) peroxydicarbonate, tert-butyl per (2-ethyl) hexanoate, tert-butyl perpivalate, tert-butyl perneodecanoate, tert-butyl perisobutyrate, tert-butyl per-3, 5, 5-trimethylhexanoate, tert-butyl perbenzoate, and dialkyl peroxides including, but not limited to, di-tert-butyl peroxide, and mixtures thereof.

Pure peroxide is typically mixed in a hydrocarbon solvent and then injected into the reactor at the injection point described herein. Any suitable pump may be used, such as a hydraulically driven piston pump.

The process of the present invention can advantageously use from 0.3kg to 1.5kg of initiator per tonne of polyethylene polymer produced and less than 0.7kg of initiator per tonne of polyethylene.

Conditioning agents

As used herein, the term "modifier" refers to a compound added to the process to control the molecular weight and/or melt index of the polymer produced. As used herein, the term "chain transfer agent" is interchangeable with the term "regulator". The modifier may be at least one of the following: tetramethylsilane, cyclopropane, sulfur hexafluoride, methane, tert-butanol, perfluoropropane, deuterated benzene, ethane, ethylene oxide, 2, 2-dimethylpropane, benzene, dimethyl sulfoxide, vinyl methyl ether, methanol, propane, 2-methyl-3-buten-2-ol, methyl acetate, tert-butyl acetate, methyl formate, ethyl acetate, butane, triphenylphosphine, methylamine, methyl benzoate, ethyl benzoate, N, N-diisopropylacetamide, 2,2, 4-trimethylpentane, N-hexane, isobutane, dimethoxymethane, ethanol, N-heptane, N-butyl acetate, cyclohexane, methylcyclohexane,1, 2-dichloroethane, acetonitrile, N-ethylacetamide, propene, N-decane, N, N-diethylacetamide, cyclopentane, acetic anhydride, N-tridecane, N-butyl benzoate, isopropanol, toluene, hydrogen, acetone, 4, 4-dimethylpenten-1, trimethylamine, N, N-dimethylacetamide, isobutylene, N-butyl isocyanate, methyl butyrate, N-butylamine, N, N-dimethylformamide, diethylsulfide, diisobutylene, tetrahydrofuran, 4-methylpentene-1, p-xylene, p-diisocynate

Alkyl, trimethylamine, butene-2, 1-bromo-2-chloroethane, octene-1, 2-methylbutene-2, cumene, butene-1, methylvinyl sulfide, n-butyronitrile, 2-methylbutene-1, ethylbenzene, n-hexadecene, 2-butanone, n-butyl isothiocyanate, methyl 3-cyanopropionate, tri-n-butylamine, 3-methyl-2-butanone, isobutyronitrile, di-n-butylamine, methyl chloroacetate, 3-methylbutene-1, 1, 2-dibromoethane, dimethylamine, benzaldehyde, chloroform, 2-ethylhexene-1, propionaldehyde, 1, 4-dichlorobutene-2, tri-n-butylphosphine, dimethylphosphine, methyl cyanoacetate, carbon tetrachloride, bromotrichloromethane, di-n-butylphosphine, acetaldehyde, phosphine, and mixtures thereof. Often the modifier is an aldehyde including acetaldehyde, propionaldehyde, butyraldehyde and mixtures thereof. In one embodiment of the invention, the modifier may be present in the invention in an amount of up to 5 kg/ton polyethylene, alternatively from 0.5 to 5 kg/ton polyethylene, alternatively from 1 to 5 kg/ton polyethylene, alternatively from 2 to 5 kg/ton polyethylene, alternatively from 3 to 5 kg/ton polyethylene, alternatively from 4 to 5 kg/ton polyethylene.

For additional details of modulators, see Advances in Polymer Science, Vol.7, pp.386-448, (1970). Table 7 therein sequences several chain transfer agents in the order of their chain transfer constants determined under the set conditions. Aldehydes (including propionaldehyde and acetaldehyde) are known to have higher chain transfer constants than other chain transfer agents (e.g., propane, butane, isobutane, propylene, isobutylene, and 1-butene).

The regulator may comprise C₂-C₂₀Or C₂-C₁₂An aldehyde. The regulator may further comprise C₂-C₂₀Or C₂-C₁₂A saturated modifier. In addition, the regulator may comprise C₂-C₂₀Or C₂-C₁₂An unsaturated modifier.

Certain embodiments and features have been described using a set of numerical upper limits and a set of numerical lower limits. It is understood that combinations from any lower limit to any upper limit are contemplated unless otherwise indicated. Certain lower limits, upper limits and ranges appear in one or more claims below. All numerical values are "about" or "approximately" the indicated value, and take into account experimental error and deviation as would be expected by one skilled in the art.

To the extent a term used in a claim is not defined above, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Moreover, all patents, test procedures, and other documents cited in this application are fully incorporated by reference herein for all jurisdictions in which such incorporation is permitted, provided that such disclosure is not inconsistent with this application.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A process for controlling the concentration of a modifier in an ethylene polymerization reactor, the process comprising:

the ethylene monomer was compressed to a pressure of 1000-3000 bar;

introducing the compressed ethylene monomer into a reactor;

introducing an initiator into the reactor; and

introducing a moderator into a plurality of moderator injection points comprising at least one front flow at the front end of the reactor and at least three side flows spaced along the length of the reactor;

wherein the amount of moderator fed at each moderator injection point is controlled independently of the amount of moderator fed in the other side streams; and

further wherein the chain transfer constant of the modifier is 0.2 or greater as measured at 1360atm and 130 ℃.

2. The method of claim 1, wherein the modulator is introduced into the plurality of modulator injection points using one modulator pump with a flow controller, the one modulator pump enabling said independent control of the amount of modulator supplied to each modulator injection point.

3. The method of claim 1, wherein the modulator is introduced into the plurality of modulator injection points using separate modulator pumps that enable said independent control of the amount of modulator supplied to each modulator injection point.

4. The method of claim 1, wherein each side stream individually comprises from 0.1 wt% to 60 wt% of the conditioning agent, based on the total weight of the material supplied through the side stream; and further wherein the amount of conditioning agent supplied individually by each sidestream differs from the amount of conditioning agent supplied individually by each other sidestream by less than 5 wt%.

5. The process of claim 1 or 4, wherein the reactor comprises a plurality of reaction zones spaced along the length of the reactor, and the concentration of modifier in the last reaction zone is maintained at or above 0.1 wt% based on the total weight of material in the last reaction zone.

6. The process of claim 1 or 4, wherein the reactor comprises a plurality of reaction zones spaced along the length of the reactor, and the concentration of modifier in each reaction zone is maintained at or above 0.1 wt% based on the total weight of materials in the reaction zone.

7. The process of claim 1 or 4, wherein the regulator and the monomer are fed to the reactor separately from each other.

8. The process of claim 1 or 4, wherein the regulator and the initiator are fed to the reactor separately from one another.

9. The process of claim 1 or 4, wherein the initiator is used in an amount of from 0.3kg to 1.5kg per ton of polyethylene polymer produced.

10. The process of claim 1 or 4, wherein the amount of initiator used is equal to or less than 0.7 kg/ton of polyethylene polymer produced.

11. The process of claim 1 or 4, wherein the reactor comprises a tubular reactor or an autoclave reactor.

12. The process of claim 1 or 4, wherein an ethylene polymer is recovered from the reactor and has a melt index of 0.1 to 3dg/min and a density of 0.913g/cm³-0.936g/cm³。